The primary technology for capturing cells is magnetic beads. In this technology, a suspension of beads is used to treat a sample containing cells. The magnetic beads contain a tag or chemical entity that is selective for cells or for a certain cell type within the sample. After the cells become associated with the magnetic beads, a magnet is used to collect the magnetic beads and captured cells. The magnetic beads may be re-suspended several times with wash solutions to clean the cells. Finally, a solution can be used to release the cells from the beads and a magnet separates the magnetic beads from the cells.
With magnetic beads, cells can be selected either positively or negatively with respect to particular antigens. With positive selection, cells expressing the antigen(s) of interest attach to magnetic beads which in turn attach to the magnetic column as described above. This method is useful for isolation of a particular cell type, for instance CD4 lymphocytes. In negative selection, antibodies are directed against surface antigen(s) present on cells that are not of interest. The undesired cells bind magnetic beads which bind the column and the fraction that goes through is collected, as it contains almost no undesired cells.
However, magnetic beads can negatively impact cell viability. This problem is mitigated somewhat by the use of magnetic nanoparticles. The magnetic-activated cell sorting (MACS) method available from Miltenyl Biotec utilizes magnetic nanoparticles to isolate cells. Cells expressing particular surface antigens attach to the magnetic nanoparticles.
The isolation of circulating tumor cells (CTCs) from blood is an area of very active interest at present. These cells which are shed into the vasculature from a primary tumor circulate in the bloodstream and constitute seeds for subsequent growth of additional tumors (metastasis) in vital distant organs. CTCs present in the bloodstream of patients with cancer provide a potentially accessible source for detection, characterization, and monitoring of non-hematological cancers.
Recently, a number of different technologies have become available for isolation and quantitation of CTCs present in blood. Some technologies exploit the physical properties of CTCs which are generally larger and stiffer than blood cells. CellSieve by Creatv Microtech and ScreenCell CC cartridge by ScreenCell perform size-based separations on blood to isolate CTCs. Although these approaches have potential, CTCs may vary in size and can be vulnerable to shear stress.
Other platforms utilize antibodies against a protein called epithelial cell adhesion molecule (EpCAM) which is found on epithelial cell tumor cells such as those found in breast, prostate and colon cancer. For example, the CellSearch system by Veridex is an FDA-approved method that uses ferrofluids loaded with EpCAM to capture CTCs. Although this system is widely accepted, the sensitivity is low, giving rise to false negatives.
There are also microfluidic devices that capture these EpCAM-expressing cells using antibody-coated micro-posts or channels. In a first generation device, 78,000 antibody-functionalized micro-posts were used to separate cells. Another microfluidic mixing device called the herringbone-chip is made up of parallel slanted channels (Li et al, Lab Chip, 13, 602). These microfluidic devices are continuing to improve.
These magnetic bead methods and chip based columns are slow and do not always produce pure cell populations. In addition, cells isolated on magnetic beads are may not be viable. There exists a need for a column technology that rapidly captures highly concentrations of cells, particularly viable cells at high concentrations and then recovers the cells at high purity.